I've been working for a certain employer for many year and I am now being treated badly what should I do?

I worked for the federal government for nearly 20 years. Two of the twenty years has was in the Marines. I am a veteran. During my civil service tenure, I have received numerous awards and letters of commendation and appreciation. I started working for this agency in Chicago in 2000 when I transferred back here to attend law school. While working here, the supervisor who has since been promoted to the program manager and his cronies who he pushed to be promoted to a supervisor position has treated me badly, has given me medicure ratings, has not provided me opportunities to use my education and skills such as computer programming and now my law degree and has purposefully not provided me with work at my grade level, which he uses to justify his rating. I have not control of the work that is being assigned to me. I've attempted to go to Informal Dispute Resolution (IDR) to discuss my most recent rating issues, but he refuses to sit down an discuss anything. Any suggestions?